Constructing Red Numbers for setting conservation priorities of endangered plant species: Israeli flora as a test case

A common problem in conservation policy is to define the priority of a certain species to invest conservation efforts when resources are limited. We suggest a method of constructing red numbers for plant species, in order to set priorities in conservation policy. The red number is an additive index, summarising values of four parameters: 1. Rarity - The number of sites (1 km²) where the species is present. A rare species is defined when present in 0.5% of the area or less. 2. Declining rate and habitat vulnerability - Evaluate the decreasing rate in the number of sites and/or the destruction probability of the habitat. 3. Attractivity - the flower size and the probability of cutting or exploitation of the plant. 4. Distribution type-scoring endemic species and peripheral populations. The plant species of Israel were scored for the parameters of the red number. Three hundred and seventy (370) species, 16.15% of the Israeli flora entered into the "Red List" received red numbers above 6. "Post mortem" analysis for the 34 extinct species of Israel revealed an average red number of 8.7, significantly higher than the average of the current red list. Only 15 species were known only from one site before extinction, indicating that rarity is not the only factor of extinction in Israeli flora. The red number suggested here is a pragmatic method and can be easily modified for conservation needs of any region. The red number method can supply a powerful quantitative weapon in the struggle for conservation.